The present invention relates to an electrostatic multipole lens used in an instrument utilizing a charged-particle beam such as a mass spectrometer.
Electrostatic multipole lenses such as electrostatic quadrupole lens, sextupole lens, and octupole lens are known as means for focusing beam of charged particles or for correcting aberrations in charged-particle beams. FIG. 1 shows an example of an electrode arrangement in an electrostatic octupole lens. In this geometry, eight cylindrical electrodes are circumscribed about a circle of radius r and equally spaced 45.degree. from each other. Voltages of +V and -V are alternately applied to the electrodes.
In this geometry, the electrodes are equally spaced from each other circumferentially on the same circle. Where it is necessary to secure a wider path of charged particles in the direction of the X axis as indicated by the broken lines, the radius r of the circle is selected according to the width of the path taken along the X axis. Space is very inefficiently utilized along the Y axis. This makes it difficult to miniaturize the lens.